This is a clinical trial testing the efficacy of a novel medication (compared to placebo), and behavioral therapy (compared to a control mechanism), in the treatment of patients with cocaine dependence. The pharmacotherapy used in this study is tryptophan, a naturally-occurring compound that has shown promising anti-cocaine effects in pre-clinical studies. The behavioral therapy is a voucher incentive program that has been shown to be effective in decreasing cocaine use in a substantial proportion of patients with cocaine dependence. The study is a 2x2 design (tryptophan/placebo, vouchers/control condition), so participants are randomly assigned to one of four study conditions. While voucher incentive programs have been used previously in outpatient clinical trials, a novel feature to this study is that the experimental interventions are studied using a relapse-prevention design - patients enrolled in the project are initially hospitalized on a residential research unit. This initial residential period allows intensive evaluation of subjects and stabilization on tryptophan or placebo under supervision of staff. In addition, the residential treatment phase ensures that all patients begin outpatient treatment after a period of cocaine abstinence. After treatment on the ward, subjects are enrolled in outpatient treatment and undergo continued treatments and intensive evaluation for the efficacy and safety of the study interventions. Following the completion of the study period, subjects are encouraged to enroll in continued outpatient substance abuse counseling.